arch: mach-k3: common: Add support to shutdown MCU R5 Core 1

arch: mach-k3: common: Add support to shutdown MCU R5 Core 1

During boot, ROM can bring up the MCU R5F cores in either lockstep or
split mode based on X509 certificate flags. If booted in split mode,
core 0 will run DM firmware and second core sits in WFI. Add support to
shut down core 1 so that other firmwares can later be loaded on them.

The shutdown of MCU R5 Core 1 is invoked at A72 SPL init, as by that
time Device Manager (DM) is up and running on R5 Core 0. The shutdown
request of Core 1 is handled by Device Manager itself.

Signed-off-by: Beleswar Padhi <b-padhi@ti.com>